1. Field of the Invention
The present invention relates to an error-correcting tandem encoding system for digital radio communications, and more particularly to an error-correcting tandem encoding system for digital radio communications including data and voice communications.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows in block form a conventional error-correcting tandem encoding system as disclosed in Japanese laid-open patent publication No. 195732/90. FIGS. 2A and 2B of the accompanying drawings show a time-series input signal of a Reed-Solomon encoder of the error-correcting tandem encoding system shown in FIG. 1 and code words that are generated thereby. FIG. 3 of the accompanying drawings shows a time-series output signal from an interleaver of the error-correcting tandem encoding system shown in FIG. 1.
The error-correcting tandem encoding system shown in FIG. 1 includes a transmitter section and a receiver section. The transmitter section comprises an input terminal 20 for receiving digital data to be transmitted, a Reed-Solomon encoder 21, an interleaver 22, a control signal input terminal 23, a selector 24, a convolutional encoder 6, a phase-shift keying (PSK) modulator 26 whose bit rate is variable by a control signal, and an output terminal 8 for outputting an intermediate-frequency (IF) signal. The receiver section comprises an input terminal 9 for receiving an IF signal, a PSK demodulator 29 whose bit rate is variable by a control signal, a Viterbi decoder 11, a deinterleaver 12, a Reed-Solomon decoder 13, a selector 33, a control signal input terminal 34, and an output terminal 35 for outputting decoded digital data.
Operation of the error-correcting tandem encoding system shown in FIG. 1 will be described below. The error-correcting tandem encoding system shown in FIG. 1 employs a convolutional code as an internal code and a Reed-Solomon code as an external code. If an input signal supplied from the input terminal 20 is a data signal of a computer, for example, the input data signal is then Reed-Solomon-encoded by the Reed-Solomon encoder 21. The Reed-Solomon-encoded digital data are block-interleaved by the interleaver 22. Specifically, a series of information symbols as shown in FIG. 2A is supplied to the Reed-Solomon encoder 21, which generates I code words 1 through I as shown in FIG. 2B. In FIG. 2B, Pi,j represents a jth check symbol for a code word i. The code word data are then supplied to and outputted from the interleaver 22 such that they are of a time series as shown in FIG. 3.
In response to a control signal from the control signal input terminal 23, the selector 24 selects and outputs the input signal from the input terminal 20 if the input signal is used in voice communications, and selects and outputs the output signal from the interleaver 22 if the input signal is used in computer communications.
When the Reed-Solomon encoder 21 and the interleaver 22 are bypassed, since no check symbol is added, the bit rate of the data inputted to the convolutional encoder 6 is varied. Therefore, the PSK modulator 26 has a variable bit rate. The bit rate of the PSK modulator 26 is varied by the control signal so as to cope with a variation in the bit rate of the supplied data. The receiver section shown in the lower half of FIG. 1 operates in a manner which is the exact reverse of the transmitter section. Therefore, operation of the receiver section will not be described below.
In the conventional error-correcting tandem encoding system, the input data pass through the Reed-Solomon encoder 21 for data communications but, in order to reduce decoding delay, the input data do not pass through the Reed-Solomon encoder 21 for voice communications. Consequently, the PSK modulator 26 of the transmitter section is required to vary its bit rate to cope with the different bit rates of the data. The PSK demodulator 29 of the receiver section is also required to vary its bit rate to cope with the different bit rates of the data. However, the conventional system is unable to satisfy the requirements for a case in which the sound quality of voice data to be received is of critical importance and a decoding delay causes no problem with respect to voice data. In addition, the conventional system fails to transmit voice and data signals simultaneously.